A solute diffuses through a stagnant liquid film 1.0 mm thick. Diffusivity is 1.0e-9 m2/s and concentration difference is 2.0 kmol/m3. Steady flux magnitude is most nearly:
- (A)0.000000002 kmol/(m^2 s)
- (B)0.000002 kmol/(m^2 s)
- (C)0.002 kmol/(m^2 s)
- (D)2 kmol/(m^2 s)
Show worked solution
Answer: (B)
Fick's law gives N=D delta C/L=2.0e-6 kmol/(m2 s).
A linear steady concentration profile gives constant diffusive flux.
The film thickness is 0.0010 m.
FE Reference Handbook — Mass Transfer: Fick's First Law
Why the other choices appear
- (A)Multiplying diffusivity by concentration difference without dividing by film thickness understates flux.
- (C)Using film thickness in millimetres as though it were metres overstates conversion.
- (D)Omitting diffusivity gives a concentration-gradient quantity, not molar flux.